Internal combustion engines may use variable cam timing (VCT) to improve fuel economy and emissions performance of a vehicle. The VCT device may include a vane type cam phaser that is controlled by an electromechanically actuated spool valve. The spool valve may direct flow of a hydraulic fluid, such as oil, from one side of the vane to the other, such as from a retard side to an advance side. The VCT device may include more than one oil circuit connecting one side of the vane to the other through which the flow of a hydraulic fluid may be directed. The phaser may be oil pressure actuated, wherein the actuation of the phaser is dependent on oil pressure in the circuit. Alternatively, the phaser may be cam torque actuated wherein the actuation of the phaser is dependent on torque generated during cam actuation.
One example of a cam torque actuated VCT phaser is shown by Smith et al. in U.S. Pat. No. 8,356,583. Therein, the VCT device is configured with a hydraulically activated locking pin in an intermediate position (herein also referred to as a mid-lock position). Conventional VCT devices may include a locking pin at one end of the range of the phaser. The VCT device of Smith also utilizes two independent oil circuits, herein referred to as the phasing circuit and the detent circuit. In the mid-lock VCT phaser of Smith, a piloted valve is included in the phaser's rotor assembly and is moveable from a first position to a second position. When the piloted valve is in the first position, hydraulic fluid is blocked from flowing through the piloted valve. When the piloted valve is in the second position, hydraulic fluid is allowed to flow between a detent line from the advance chamber and a detent line from the retard chamber through the piloted valve and a common line, such that the rotor assembly is moved to and held in the intermediate phase angle position relative to the housing assembly. Detent lines communicating with the advance chamber or retard chamber are blocked when the VCT phaser is at or near the intermediate position. The spool valve has three regions of operation, namely Detent (or Auto-Lock), Retard, and Advance in the specified order. Specifically, when the spool valve is commanded to the retard or advance regions, the piloted valve is in the first position, and fluid is blocked from flowing through the detent circuit lines. Additionally, fluid may flow from one side of the vane to the other via the phasing circuit lines. When the spool valve is commanded to the detent region, the piloted valve is in the second position, and fluid is free to flow from the advanced or retarded chamber, through the detent lines and the piloted valve, and into the opposite chamber through a common fluid line. Additionally, fluid is blocked from flowing through the phasing circuit lines.
However, the inventors herein have identified potential issues with such a VCT system. While the spool valve may ideally have three regions of operation (namely detent, retard, and advance), in practice, there may be additional areas of operation existing at the border of any two consecutive regions where the two regions are simultaneously active. For example, additional areas may exist at the border of the detent and retard regions, and likewise at the border of the retard and advance regions. In the event that the spool valve is commanded to a position between the detent and retard regions, the detent circuit and phasing circuit may compete for hydraulic control of the cam phaser position. As a result, the cam phaser may be locked when a command to retard the cam phaser position was intended. In another scenario, the phaser may not predictably respond to spool valve commands due to additional and erratic actuation via the flow of fluid through detent circuit lines. Further still, the cam phaser may be retarding when a command to auto-lock the cam phaser was intended. As such, any of these scenarios may result in engine performance degradation.
In one example, the issues described above may be addressed by a method comprising: during selected conditions, ramping a spool valve coupled to a cam torque actuated variable cam timing phaser from a detent region to a retard region; and mapping a transitional region between the detent and retard regions based on phaser movement away from a locked position, the phaser movement responsive to the ramping. In this way, command of the spool valve into the region where both the detent circuit and retard circuit line are active is avoided.
As an example, a map of circuit operation as a function of solenoid duty cycle value may be generated during phaser operation. In particular, a boundary between the phasing regions and the detent region of spool valve operation may be defined from this mapping. By performing the mapping intermittently, during selected conditions, the boundaries may also be updated as phaser conditions change. For example, the highest duty cycle value at which the detent circuit is active may be identified as the lower limit of the phasing region. Additionally, the lowest duty cycle for which retarding motion is detected may be identified as the upper limit of duty cycle commands in the detent region. These boundary values may be adaptively updated during phaser operation using adaptive learning algorithms. For instance, if upon a new mapping, the lowest duty cycle for which retarding motion is detected is different from the previously stored value, the boundary value may be updated as a function of the most recently determined value, and the map may be updated.
In this way, a boundary between spool valve regions may be learned during phaser operation based on phaser motion. By learning the boundary more accurately, phaser operation may be improved. By iteratively learning and updating the boundaries in relation to the spool valve map, adaptable boundaries may be placed on acceptable regions of duty cycle command. Consequently, duty cycle commands can be adjusted so as not to enter a region where competition for hydraulic control of cam phaser position can occur. By more accurately mapping the boundary between spool valve regions, phaser response to spool valve commands may be rendered more consistent. In addition, unintended and undesired cam phaser positioning may be averted.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.